[enhancement] Support X-Script-Name header for the admin webpage

[efficks]

I'm running carbon-relay-ng behind a reverse proxy with the root URL /carbon-relay-ng
Carbon-relay-ng does not return it's internal URL with the /carbon-relay-ng prefix when the HTTP Header X-Script-Name is set.

I want to be able to run carbon-relay-ng behind a proxy with a prefix url by setting the X-Script-Name header.

Thank you